Dairy cows are significant investments for dairy farmers, and enormous efforts, such as animal breeding and artificial insemination, have been and continue to be invested in ensuring that the animals have high and sustained productivity, and that the milk produced are of high quality. Traditional breeding techniques involve the studying of sire progenies, and evaluating their traits including milk production ratings (transmitting abilities) to guide further breeding. This standard technique is time consuming and costly, requiring years to evaluate the true genetic value by progeny testing each bull. Many cows must be bred and give birth to offspring. The females must be raised, bred, allowed to give birth and finally milked for a length of time to measure their phenotypic traits.
Furthermore, selection based purely on phenotypic characteristics does not efficiently take into account genetic variability caused by complex gene action and interactions, and the effect of the environmental and developmental variants. There is thus a need for a method of genetically evaluating cattle to enable breeders to more accurately select animals at both the phenotypic and the genetic level.
Marker-assisted selection can lower the high cost of progeny testing currently used to improve sires, since young bull progeny could be evaluated immediately after birth, and young bulls that are determined by genetic testing to have undesirable markers would never be progeny tested. Testing may even be conducted prior to birth, for the presence/absence of the marker. Therefore, there is also a need for genetic markers for improved milk production traits.
POU1F1 is a member of the tissue specific POU (Pit, Oct, Unc) homeobox transcription factor DNA binding protein family that is found in all mammals studied so far (Bastos et al., 2006; Ingraham et al., 1988; Ingraham et al., 1990). The pituitary specific expression of POU1F1 is required for the activation of growth hormone (GH), prolactin (PRL), and thyroid stimulating hormone (TSH) (Li et al., 1990). These genes are involved in a variety of signaling pathways that are important for many developmental and physiological processes, including pituitary gland development (Li et al., 1990, Mullis, 2007), mammary gland development and growth (Svennersten-Sjaunja and Olsson, 2005), milk protein expression (Akers, 2006), and milk production and secretion (Svennersten-Sjaunja and Olsson, 2005). Moreover, binding of GH and PRL to their receptors on the cell membrane triggers a cascade of signaling events including the JAK/STAT pathway, which has been shown to be required for adult mammary gland development and lactogenesis (Liu et al., 1997).
Mutations in POU1F1 often result in severe GH deficiency as well as defects in development (Mullis, 2007). In a dwarf mouse model, mutations in POU1F1 lead to the loss of three pituitary cell types—somatotropes, lactotropes and thyrotropes—(Li et al., 1990). Lactotropes produce prolactin, which is necessary for mammary gland development and lactation.
Several genes in the same pathway of POU1F1 have been reported to be associated with different milk production and health traits. For example, growth hormone receptor (GHR) and prolactin receptor (PRLR) have shown associations with milk yield and composition (Viitala et al., 2006). Also, the signal transducer and activator of transcription 1 (STAT1) and osteopontin (OPN) genes have been shown to have significant effects on milk yield and milk protein and fat yields in Holstein dairy cattle (Cobanoglu et al., 2006; Leonard et al., 2005; Schnabel et al., 2005). The uterine milk protein (UTMP) is another gene in the pathway of POU1F1 that has been found to be associated with productive life in dairy cattle (Khatib et al., 2007b).
POU1F1 is located on bovine chromosome region BTA1q21-22 (Woollard et al., 2000), where multiple quantitative trait loci (QTL) affecting milk production traits have been identified (Georges et al., 1995; Nadesalingam et al., 2001). In previous studies, POU1F1 variants have been reported to be associated with milk yield and conformation traits (Renaville et al., 1997; Tuggle and Freeman, 1994). Taken together, the biological functions of POU1F1 and associations with production traits of genes in the same pathway of POU1F1 suggest that this gene could be functionally involved in milk yield and composition traits.